Hi WHBM,
The blocked seats have everything to do with center of gravity (as explained above) in combination with the free seating policy. In another airline you may get a boarding card with a seat number. Now the computer has allocated seats and makes sure that everyone gets spread out evenly throughout the aircraft cabin, keeping the center of gravity within limits.
With Ryanair's 'sit where you like' policy, it's theoretically possible -if the plane isn't full- that all passengers will sit towards the front or towards the back of the plane. Basically the only way to keep this situation under control is to block of certain rows. Sometimes 2 rows will be blocked off, sometimes 6 in the front and 4 in the back. It all depends on what the loadsheet requires: it depends on passenger numbers, number of bags and in which holds they are stored and the amount of fuel carried. This makes it impossible to judge from a pax point of view without seeing the actual loadsheet.
This procedure has been designed in cooperation with the aircraft's manufacturer Boeing.
The relevance of the 25 min turnaround quoted above? Free seating policy makes people 'run' for the best seats so boarding goes quicker.
With respect to the reduction in baggage allowance that you mentioned; isn't that reduction compensated by an increase in handluggage allowance?
Hope this helps!
Best regards,
FC